1. Field of the Invention
This invention relates to a video display control system for use as terminal equipment for a computer or video machines.
2. Prior Art
There has recently been proposed a video display control system which comprises a video display controller, a central processing unit (CPU), a video RAM (VRAM) and a CRT display unit and displays a color video image composed of a plurality of display elements on a screen of the CRT display unit in accordance with color codes read by the video display controller from the VRAM. There has been known among such systems, one in which each color code is composed of, for example, four bits for selecting one of sixteen colors, and wherein one of the sixteen color codes (for example one represented by "0, 0, 0, 0") is assigned to transparency. In such system, when a color code representative of transparency is read from the RAM, a display element on the screen corresponding to the read color code is displayed in a predetermined backdrop color or in a color of the corresponding display element of the backdrop image.
The color code representative of transparency is particularly useful to superimpose a video image over another video image but is rarely used in display processing other than the superimpose processing. It is therefore desirable to use the color code representative of transparency as a color code representative of an additional color in the case where the superimpose processing is not carried out. However, with the conventional system in which each color code is composed of, for example, four bits, a specific one of the sixteen color codes is used as the color code representative of transparency as described above, so that only fifteen colors can be designated by the color codes.